Iron-based nanoparticles and grains

ABSTRACT

Example nanoparticles may include an iron-based core, and a shell. The shell may include a non-magnetic, anti-ferromagnetic, or ferrimagnetic material. Example alloy compositions may include an iron-based grain, and a grain boundary. The grain boundary may include a non-magnetic, anti-ferromagnetic, or ferrimagnetic material. Example techniques for forming iron-based core-shell nanoparticles may include depositing a shell on an iron-based core. The depositing may include immersing the iron-based core in a salt composition for a predetermined period of time. The depositing may include milling the iron-based core with a salt composition for a predetermined period of time. Example techniques for treating a composition comprising core-shell nanoparticles may include nitriding the composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/340,031, filed April 5, 201, which is a National Stage ofInternational Patent Application No. PCT/US2017/055531, filed Oct. 6,2017, and claims the benefit of U.S. Provisional Patent Application No.62/405,661, filed Oct. 7, 2016, and titled “IRON-BASED NANOPARTICLES ANDGRAINS,” the entire contents of which are incorporated by referenceherein.

GOVERNMENT INTEREST

This invention was made with Government support under contract numberDE-AR0000199 awarded by the DOE, Office of ARPA-E. The government hascertain rights in this invention.

TECHNICAL FIELD

The disclosure relates to iron-based nanoparticles and grains andtechniques for forming iron-based nanoparticles and grains.

BACKGROUND

Permanent magnets play a role in many electromechanical systems,including, for example, alternative energy systems. For example,permanent magnets are used in sensors, actuators, electric motors orgenerators, which may be used in vehicles, wind turbines, and otheralternative energy mechanisms. Many permanent magnets in current useinclude rare earth elements, such as neodymium, which result in highenergy product. These rare earth elements are in relatively shortsupply, and may face increased prices and/or supply shortages in thefuture. Additionally, some permanent magnets that include rare earthelements are expensive to produce. For example, fabrication of NdFeB andferrite magnets generally includes crushing material, compressing thematerial, and sintering at temperatures over 1000° C., all of whichcontribute to high manufacturing costs of the magnets. Additionally, themining of rare earth can lead to severe environmental deterioration.

Iron nitride magnets based on the Fe₁₆N₂/Fe₈N phase are of interest as amagnetic material for applications ranging from data storage toelectrical motors for vehicles, wind turbines, and other powergeneration equipment. The component base elements (Fe, N) areinexpensive and widely available, in contrast to rare earth elements inrare earth element-based magnets, which are costly and subject to supplyavailability risks. The Fe₁₆N₂ phase, which is the ordered version ofFe₈N, has a large magnetic anisotropy constant and saturationmagnetization but is difficult to manufacture.

SUMMARY

The disclosure describes example nanoparticles. In some examples, thenanoparticles may include an iron-based core, and a shell. The shell mayinclude a non-magnetic material, an anti-ferromagnetic material, or aferromagnetic material.

The disclosure also describes example alloy compositions. In someexamples, the alloy compositions may include an iron-based grain, and agrain boundary. The grain boundary may include a non-magnetic material,an anti-ferromagnetic material, or a ferromagnetic material.

The disclosure describes example techniques for forming iron-basedcore-shell nanoparticles. In some examples, the core-shell nanoparticlesmay be formed by a technique including depositing a shell on aniron-based core by at least immersing the iron-based core in a saltcomposition for a predetermined period of time. The shell may include anon-magnetic material, an anti-ferromagnetic material, or aferromagnetic material.

In some examples, the core-shell nanoparticles may be formed by atechnique including depositing a shell on an iron-based core by at leastmilling the iron-based core with a salt composition for a predeterminedperiod of time. The shell may include a non-magnetic material, ananti-ferromagnetic material, or a ferromagnetic material.

The disclosure describes example techniques for treating a compositioncomprising a core-shell nanoparticle. The core-shell nanoparticleincludes an iron-based cores and a shell including a non-magneticmaterial, an anti-ferromagnetic material, or a ferrimagnetic material.The examples techniques include nitriding the composition. In someexamples, the nitriding may include autoclaving the composition in anitrogen-rich environment or milling the composition with a nitrogensource.

The disclosure describes example techniques for preparing a core-shellnanoparticle. The technique includes nitriding a composition comprisingan iron-based core to form a shell comprising iron nitride on theiron-based core.

The disclosure describes example techniques for treating a compositioncomprising an iron-based material. The technique includes nitriding thecomposition by exposing the composition to a predetermined pressure, ata predetermined temperature, for a predetermined period of time, in anenvironment including a nitrogen source.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a conceptual and schematic diagram illustrating an examplecore-shell nanoparticle.

FIG. 1B is a conceptual and schematic diagram illustrating an exampleellipsoidal core-shell nanoparticle.

FIG. 2A is a conceptual and schematic diagram illustrating acrystallographic unit cell of α-Fe.

FIG. 2B is a conceptual and schematic diagram illustrating acrystallographic unit cell of α″-Fe₁₆N₂.

FIG. 3 is a conceptual and schematic diagram illustrating amicrostructure including grains and grain boundaries of an example alloycomposition.

FIG. 4 is a conceptual and schematic diagram illustrating an exampleimmersion system for forming a core-shell nanoparticle by treatment of acore nanoparticle with a salt composition.

FIG. 5 is a flow diagram illustrating an example technique for forming acore-shell nanoparticle by treating a core nanoparticle with a saltcomposition.

FIG. 6 is a conceptual and schematic diagram illustrating an examplemilling system for forming a core-shell nanoparticle by milling.

FIG. 7 is a flow diagram illustrating an example technique for forming acore-shell nanoparticle by milling.

FIG. 8 is a flow diagram illustrating an example technique for treatinga composition including an iron-based material.

FIG. 9 is a diagram illustrating a hysteresis loop of magnetizationversus applied magnetic field at room temperature for examplenanoparticles including Fe₁₆N₂ cores and MnN shells.

FIG. 10 is a diagram illustrating x-ray diffraction data for examplenanoparticles including Fe cores and Fe₁₆N₂ shells formed by nitridingFe nanoparticles.

FIG. 11 is a diagram illustrating a hysteresis loop of magnetizationversus applied magnetic field at room temperature for the examplenanoparticles of FIG. 10.

FIG. 12 is a diagram illustrating x-ray diffraction data for examplenanoparticles including Fe cores and Fe₁₆N₂ shells formed by nitridingFe nanoparticles.

FIG. 13 is a diagram illustrating a hysteresis loop of magnetizationversus applied magnetic field at room temperature for the examplenanoparticles of FIG. 12.

FIG. 14 is a diagram illustrating the effect of field annealing on ahysteresis loop of magnetization versus applied magnetic field at roomtemperature for example nanoparticles including FeCo cores and MnNshells.

FIG. 15 is a diagram illustrating a hysteresis loop of magnetizationversus applied magnetic field at room temperature and a low temperaturefor example nanoparticles including Fe cores.

FIG. 16 is a diagram illustrating a hysteresis loop of magnetizationversus applied magnetic field at room temperature and at a lowtemperature for example nanoparticles including Fe cores and MnN shells.

DETAILED DESCRIPTION

The disclosure describes core-shell nanoparticles including aniron-based core (for example, comprising elemental iron, an iron andnitrogen alloy or compound, an iron and cobalt alloy or compound, or thelike), and a shell including a non-magnetic material, ananti-ferromagnetic material, or a ferrimagnetic material, and techniquesfor preparing core-shell nanoparticles.

Ferromagnetic materials include materials in which magnetic domains arealignable into a substantially unidirectional alignment that persists inthe absence of an applied or external magnetic field. Anti-ferromagneticmaterials include materials that include substantially equal amounts ofdomains that are anti-parallel, canceling the net magnetic moment tozero. Ferrimagnetic materials include materials that includeanti-parallel domains, yet retain a net magnetic moment because a subsetof domains has a greater magnetic moment than the rest of the domains.Non-magnetic, or non-ferromagnetic materials include materials thatcannot be magnetized whether in the presence or absence of an appliedmagnetic field.

Example core-shell nanoparticles according to the disclosure may be usedto prepare bulk magnetic materials, such as bulk permanent magnets. Forexample, core-shell nanoparticles described herein may be used in, forexample, bonding magnets, pressed magnets, other bulk magnets thatinclude or do not include binder material, or the like. Without beingbound by theory, coercivity is an extrinsic property of a magneticmaterial, and is related to the microstructure. Therefore, themicrostructure, for example, grain sizes, phases, and grain boundaries,may influence coercivity of a material. For example, shells onnanoparticle cores may increase the coercivity of compositions thatinclude the nanoparticles by reducing the packing density of thenanoparticles or by increasing the grain separation or grain boundarythickness in a material that includes the core-shell nanoparticles.

Without being bound by theory, three types of anisotropy may contributeto the magnetic anisotropy energy or magnetic anisotropy field. Thesethree types of anisotropy include strain anisotropy, magnetocrystallineanisotropy, and shape anisotropy. Strain anisotropy may be related tostrain exerted on iron-based magnetic materials, for example, materialsincluding an iron nitride phase such as an α″-Fe₁₆N₂ phase. Differencesin coefficients of thermal expansion between different phases, crystals,between cores and shells, or between grains and grain boundaries mayintroduce strain due to differential dimensional changes in theparticles and the grains of iron or other types of iron nitride duringthermal processing. For example, differences between the respectivethermal expansion coefficients of the core and the shell may lead tostrain (stress-induced) anisotropy. In some examples, strain anisotropymay promote the formation of magnetic phases, for example, iron nitridephases. In some examples, shells may include one or more of siliconnitride (Si₃N₄), aluminum nitride (AlN), or zinc oxide (ZnO) to promotestrain anisotropy.

In some examples, other forms of anisotropy may be induced to increasecoercivity. For example, using an anti-ferromagnetic shell may induceexchange anisotropy, a form of magnetic anisotropy, which may increasecoercivity. In some examples, the difference between the respectivemagnetizations of the core and the shell may itself increase coercivity.For example, while the core is ferromagnetic, the shell may include anon-ferromagnetic material, a ferromagnetic magnetic material, or ananti-ferromagnetic material, and this difference may result in exchangeanisotropy.

In some examples, magnetocrystalline anisotropy may result from thecrystalline structure of phase domains within crystals. For example,magnetocrystalline anisotropy may be related to the distortion of abody-centered-cubic iron crystalline lattice into abody-centered-tetragonal iron-nitride crystalline lattice in an ironnitride crystal. Iron nitride, has a relatively high saturationmagnetization and a relatively high energy product, for example, as highas 130 MGOe.

Shape anisotropy may be related to the shape of the nanoparticles. Forexample, a nanoparticle may define a longest dimension and a shortestdimension, and the differences in these dimensions may ultimatelycontribute to magnetic anisotropy. One or more of strain, magnetic,exchange, and shape anisotropies may be used to enhance magneticproperties, such as coercivity, of nanoparticles according to thedisclosure.

The disclosure describes example techniques for preparing core-shellnanoparticles. An example technique includes depositing a shell on aniron-based core. For example, depositing the shell may include immersingthe iron-based core in a salt composition for a predetermined period oftime. The shell may deposit on the iron-based core from the saltcomposition. In some examples, depositing the shell may include millingthe iron-based core with a salt composition for a predetermined periodof time. The shell may deposit on the iron-based core from the saltcomposition. In some examples, the example techniques may includenitriding the nanoparticles, before or after the shell is deposited onthe core. For example, the nanoparticles may be nitrided by autoclavingthe nanoparticles at a predetermined pressure, at a predeterminedtemperature, for a predetermined period of time, in a nitrogen-richenvironment. In some examples, the predetermined pressure may berelatively high, such as greater than about 100 atmospheres, which mayincrease a rate of diffusion of nitrogen into the nanoparticles andreduce a time utilized for nitriding the nanoparticles.

While nitriding may be used to treat core-shell nanoparticles, thedisclosure describes example techniques in which nitriding may be usedto prepare core-shell nanoparticles. For example, an example techniquefor preparing core-shell nanoparticles may include nitriding acomposition comprising an iron-based core to form a shell comprisingiron nitride on the iron-based core.

The core-shell nanoparticles, alloy compositions, and techniquesdescribed herein may be used to form bulk magnetic materials, such asbulk permanent magnets. For example, the techniques described herein forforming material comprising core-shell nanoparticles including ironnitride may be used in processes to form iron nitride bulk permanentmagnets described in International Patent Application NumberPCT/US2012/051382, filed on Aug. 17, 2012, and titled “IRON NITRIDEPERMANENT MAGNET AND TECHNIQUE FOR FORMING IRON NITRIDE PERMANENTMAGNET;” and International Patent Application Number PCT/US2014/015104,filed on Feb. 6, 2014, and titled “IRON NITRIDE PERMANENT MAGNET ANDTECHNIQUE FOR FORMING IRON NITRIDE PERMANENT MAGNET;” and U.S.Provisional Patent Application No. 61/935,516, filed Feb. 4, 2014, andtitled “IRON NITRIDE MATERIALS AND MAGNETS INCLUDING IRON NITRIDEMATERIALS,” the entire contents of which are incorporated herein byreference.

Example techniques and core-shell nanoparticles according to thedisclosure may be used to eventually prepare bulk permanent magnetshaving relatively enhanced magnetic properties such as relatively highcoercivity. For example, permanent magnets prepared from examplecore-shell nanoparticles according to the disclosure may exhibitmagnetic properties comparable to or better than those of rare-earthmagnets, without including any rare-earth elements.

FIG. TA is a conceptual and schematic diagram illustrating an examplecore-shell nanoparticle. Example nanoparticle 10 a includes aniron-based core 12 a, and a shell 14 a. In some examples, iron-basedcore 12 a may be ferromagnetic, and have a high saturationmagnetization. Shell 14 a may substantially coat or surround iron-basedcore 12 a. Shell 14 a may include a non-magnetic material, ananti-ferromagnetic material, or a ferromagnetic material. In otherexamples, shell 14 a may include a ferromagnetic material. Withoutwishing to be bound by theory, the difference between one or both of themagnetization and thermal expansion coefficient of iron-based core 12 aand shell 14 a, and the reduction in packing density of a compositioninclude nanoparticle 10 a because of the presence of shell 14 a spacingiron-based core 12 a from a neighboring core, may ultimately promote arelatively increased coercivity of materials including nanoparticle 10a, compared to materials including nanoparticles without shell 14 a.

In some examples, iron-based core 12 a may include one or more ofelemental iron, iron nitride, α″-Fe₁₆N₂, or α′-Fe(Co)(N). In someexamples, the elemental iron may include an α-Fe phase. The iron nitridemay include any iron nitride, including one or more of Fe₂N, Fe₃N,Fe_(x)N_(1-x), where x is a number greater than 0 and less than 1, Fe₄N,Fe₇N₃, Fe₈N, and Fe₁₆N₂. The α′-Fe(Co)(N) may include an α′-Fe phaseincluding Co and N. In some examples, core 12 a may have a majordimension between about 20 nm and about 100 nm. While nanoparticle 10 ais illustrated as a spherical particle in FIG. TA, in other examples,nanoparticle 10 a may have any suitable shape. For example, nanoparticle10 a may have a spheroidal, ellipsoidal, cuboidal, polygonal, or anyother suitable shape. FIG. 1B is a conceptual and schematic diagramillustrating an example ellipsoidal core-shell nanoparticle 10 b.Nanoparticle 10 b includes an iron-based core 12 b that may have acomposition substantially similar to a composition described withreference to iron-based core 12 a of FIG. 1A, and a shell 14 b. In someexamples, iron-based core 12 b may be an ellipsoid (e.g.,three-dimensional or plate-like). In some examples, iron-based core 12 bmay be an ellipsoid with a ratio of a longitudinal diameter to atransverse diameter of at least about 2. For example, the ratio of thelongitudinal diameter to the transverse diameter may be at least about3, or about 5, or about 10. In some examples, the ratio of thelongitudinal diameter to the transverse diameter may be about 2, orabout 5. In some examples, nanoparticle may exhibit a shape anisotropy,for example, as a result of the ellipsoidal shape. In some examples, theshape anisotropy may contribute to increased magnetic anisotropy.

In some examples, shell 14 a or 14 b may include one or more ofnonmagnetic materials, such as silica, aluminum oxide, silicon nitride,aluminum nitride, zinc oxide or anti-ferromagnetic materials, such asmanganese nitride, or ferromagnetic materials, such as iron oxide, orferromagnetic materials, such as Fe₄N, and Fe₁₆N₂. In some examples,shell 14 a may have a thickness between about 1 nm and about 10 nm. Forexample, shell 14 a may have a thickness of about 1 nm, or about 5 nm,or about 10 nm.

In some examples, iron-based cores 12 a or 12 b may include Fe₁₆N₂. Insome examples, iron-based cores 12 a or 12 b may include an α″-Fe₁₆N₂phase. Throughout this disclosure, the terms Fe₁₆N₂, α″-Fe₁₆N₂,α″-Fe₁₆N₂ phase, and α″-Fe₁₆N₂ phase domain, for example, may be usedinterchangeably to refer to a α″-Fe₁₆N₂ phase domain within a material.The α″-Fe₁₆N₂ phase may exhibit an intrinsic magnetocrystallineanisotropy, as discussed with reference to FIGS. 2A and 2B.

FIG. 2A is a conceptual and schematic diagram illustrating a unitcrystallographic cell of α-Fe. FIG. 2A shows a unit cell including ironatoms 16 in an isotropic arrangement. FIG. 2B is a conceptual andschematic diagram illustrating a unit crystallographic cell ofα″-Fe₁₆N₂. FIG. 2B shows eight (8) iron unit cells in a strained statewith nitrogen atoms 18 in interstitial spaces between iron atoms 16 toform the Fe₁₆N₂ iron nitride unit cell. As shown in FIG. 2B, in theα″-Fe₁₆N₂ phase, nitrogen atoms 18 are aligned along the (002) (iron)crystal planes. The iron nitride unit cell is distorted such that thelength of the unit cell along the <001> axis is approximately 6.28angstroms (Å) while the length of the unit cell along the <010> and<100> axes is approximately 5.72 Å. The α″-Fe₁₆N₂ unit cell may bereferred to as a body-centered tetragonal unit cell when in the strainedstate. When the α″-Fe₁₆N₂ unit cell is in the strained state, the <001>axis may be referred to as the c-axis of the unit cell. The c-axis maybe the magnetic easy axis of the α″-Fe₁₆N₂ unit cell. In other words,α″-Fe₁₆N₂ crystals exhibit magnetic anisotropy. In some examples,core-shell nanoparticles 10 a or 10 b may have at least one Fe₁₆N₂ ironnitride crystal. In some examples, such an anisotropic particle mayinclude a plurality of iron nitride crystals, at least some (or all) ofwhich are Fe₁₆N₂ crystals.

The α″-Fe₁₆N₂ phase has high saturation magnetization and magneticanisotropy constant. The high saturation magnetization and magneticanisotropy constants result in a magnetic energy product that may behigher than rare earth magnets. For example, experimental evidencegathered from thin film α″-Fe₁₆N₂ permanent magnets suggests that bulkFe₁₆N₂ permanent magnets may have desirable magnetic properties,including an energy product of as high as about 130 MegaGauss*Oerstads(MGOe), which is about two times the energy product of NdFeB (which hasan energy product of about 60 MGOe). Additionally, iron and nitrogen areabundant elements, and thus are relatively inexpensive and easy toprocure.

In some examples, nanoparticles 10 a or 10 b may have a relatively highcoercivity. For example, nanoparticles 10 a or 10 b may have acoercivity of at least about 600 Oe. In some examples, nanoparticles 10a or 10 b may have a coercivity of at least about 1000 Oe. For example,nanoparticles 10 a or 10 b may have a coercivity of about 1000 Oe.

In some examples, nanoparticles 10 a or 10 b may include at least one ofFeN, Fe₂N (e.g., ξ-Fe₂N), Fe₃N (e.g., ε-Fe₃N), Fe₄N (e.g., γ′-Fe₄N),Fe₂N₆, Fe₈N, Fe₁₆N₂ (e.g., α″-Fe₁₆N₂), or FeN_(x) (where x is betweenabout 0.05 and about 0.5). Additionally, in some examples, thenanoparticles 10 a or 10 b may include other materials, such aselemental iron, cobalt, nickel, dopants, or the like. In some examples,the cobalt, nickel, dopants, or the like may be at least partiallyremoved after the milling process using one or more suitable techniques.Dopants within the nanoparticles may include, for example, at least oneof aluminum (Al), manganese (Mn), lanthanum (La), chromium (Cr), cobalt(Co), titanium (Ti), nickel (Ni), zinc (Zn), a rare earth metal, boron(B), carbon (C), phosphorous (P), silicon (Si), or oxygen (O).

Compositions, for example, mixtures, including example nanoparticles 10a or 10 b may be compacted and shaped or otherwise further processed toform bulk magnetic materials, such as permanent magnets. For example,example alloy compositions may include nanoparticles 10 a or 10 b. FIG.3 is a conceptual and schematic diagram illustrating a microstructure ofan example alloy composition 20 including grain boundaries 22 betweeniron-based grains 24. In some examples, iron-based grains 24 includematerial substantially similar to that of iron-based cores 12 a or 12 bdescribed with reference to FIGS. 1A and 1B. In some examples, grainboundaries 22 include material substantially similar to that of shells14 a and 14B described with reference to FIGS. 1A and 1B. In someexamples, example alloy composition 20 may be prepared by compacting oneor both of nanoparticles 10 a or 10 b. In other examples, alloycomposition 20 may be prepared by any suitable techniques forengineering the compositions or phase constitutions of grains and grainboundaries, or the microstructure of alloy composition 20, including forexample, casting, annealing, and nitriding. In some examples, alloycomposition 20 may be further processed, for example, by one or more ofmolding, compacting, pressurizing, or annealing, to prepare bulkmagnetic materials, such as permanent magnets. Thus, example core-shellnanoparticles and example alloy compositions according to the disclosuremay be used to prepare bulk magnetic materials, such as permanentmagnets.

Example systems and techniques described with reference to FIGS. 4-8 maybe used to prepare example core-shell nanoparticles according to thedisclosure. FIG. 4 is a conceptual and schematic diagram illustrating anexample system 30 for forming a core-shell nanoparticle by treatment ofa core nanoparticle with a salt composition 34. System 30 may include acontainer 31. Container 31 can be any suitable rigid, semi-rigid, orflexible container dimensioned to contain a sample holder 32 and saltcomposition 34. Sample holder 32 holds a composition includingiron-based core 12 a or 12 b. For example, iron-based core 12 a or 12 bincludes one or more of iron, iron nitride, α″-Fe₁₆N₂, or α′-Fe(Co)(N).In some examples, salt composition 34 includes one or more precursors ofat least one of silica, aluminum oxide, silicon nitride, aluminumnitride, manganese nitride, zinc oxide, iron oxide, ferromanganese,Fe₄N, or Fe₁₆N₂. For example, salt composition 34 may include one ormore of a liquid, a solution, a polymer, or a gel. The salt composition34 deposits shell 14 a or 14 b on core 12 a or 12 b, when core 12 a or12 b is immersed in salt composition 34 for a predetermined amount oftime. In some examples, instead of a container 31, system 30 may spray,coat, or otherwise immerse sample holder 32 in a static batch or amoving flow of salt composition 34.

FIG. 5 is a flow diagram illustrating an example technique for forming acore-shell nanoparticle by treating iron-based core 12 a or 12 b withsalt composition 34. The example technique of FIG. 5 is described withreference to system 30 of FIG. 4. However, the example technique of FIG.5 may be implemented using any suitable system.

In some examples, the example technique of FIG. 5 may optionally includereducing an iron precursor to form iron-based cores 12 a or 12 b (40).In some examples, the iron precursor may include, for example, at leastone of iron (Fe), FeCl₃, Fe₂O₃, or Fe₃O₄. In some examples, the ironprecursor may include a bulk or powder sample including Fe, FeCl₃, oriron (e.g., Fe₂O₃ or Fe₃O₄), or combinations thereof. In some examples,the precursor may include a powder including particles.

Reducing the precursor may include reducing or removing oxygen contentin the precursor. For example, an oxygen reduction process can becarried out by exposing the precursor to hydrogen gas. The hydrogen mayreact with any oxygen present in the precursor, removing oxygen. In someexamples, such a reduction step may form substantially pure iron withinthe precursor including iron (e.g., iron with less than about 10 at. %dopants). Additionally, or alternatively, reducing the precursor mayinclude using an acid cleaning technique. For example, dilutedhydrochloric acid, with a concentration between about 5% by volume andabout 50% by volume can be used to wash oxygen from the precursor toform iron-based core 12 a or 12 b.

In some examples, the example technique of FIG. 5 may optionally includenitriding iron-based core 12 a or 12 b (42). Nitriding iron-based core12 a or 12 b (42) may form an iron nitride phase in iron-based core 12 aor 12 b, and may proceed in any one of a number of manners. In general,nitrogen from a nitrogen source is combined with iron to form ironnitride. Such a nitrogen source may be the same as or similar tonitrogen sources described in elsewhere in this disclosure, such as atleast one of ammonia, ammonium nitrate, an amide-containing material, ora hydrazine-containing material.

In some examples, nitriding iron-based core 12 a or 12 b 42 may includeheating iron-based core 12 a or 12 b to a selected temperature for atime sufficient to allow diffusion of nitrogen to a predeterminedconcentration substantially throughout a volume including iron. In thismanner, the heating time and temperature are related, and may also beaffected by the composition and/or geometry of the volume includingiron. For example, the heating may include heating to a temperaturebetween about 125° C. and about 600° C. for between about 2 hours andabout 9 hours.

In addition to heating the anisotropic particle including iron,nitriding the anisotropic particle including iron may include exposingto an atomic nitrogen substance, which diffuses into the volumeincluding iron. In some examples, the atomic nitrogen substance may besupplied as diatomic nitrogen (N₂), which is then separated (cracked)into individual nitrogen atoms. In other examples, the atomic nitrogenmay be provided from another atomic nitrogen precursor, such as ammonia(NH₃). In other examples, the atomic nitrogen may be provided from urea(CO(NH₂)₂). The nitrogen may be supplied in a gas phase alone (e.g.,substantially pure ammonia or diatomic nitrogen gas) or as a mixturewith a carrier gas. In some examples, the carrier gas is argon (Ar).

In some examples, nitriding the anisotropic particle including iron mayinclude a urea diffusion process, in which urea is utilized as anitrogen source (e.g., rather than diatomic nitrogen or ammonia). Urea(also referred to as carbamide) is an organic compound with the chemicalformula CO(NH₂)₂. Urea may be heated, e.g., within a furnace enclosingthe anisotropic particle including iron, to generate decomposed nitrogenatoms which may diffuse into the volume including iron. In someexamples, the constitution of the resulting nitrided iron material maycontrolled to some extent by the temperature of the diffusion process aswell as the ratio (e.g., the weight ratio) of the iron-containingworkpiece to urea used for the process. Further details regarding thesenitriding processes (including urea diffusion) may be found inInternational Patent Application No. PCT/US12/51382, filed Aug. 17,2012, the entire content of which is incorporated herein by reference.

In some examples, nitriding iron-based core 12 a or 12 b (42) includesautoclaving iron-based cores 12 a or 12 b at a predetermined pressure,at a predetermined temperature, for a predetermined period of time, in anitrogen-rich environment, for example, using example autoclavenitriding techniques described elsewhere in the disclosure. In someexamples, the predetermined pressure may be greater than about 100atmospheres, or at least about 100 atmospheres. Without wishing to bebound by theory, diffusion of nitrogen species increases with pressure.Increasing the pressure, increases nitrogen diffusion. Using a pressureof at least about 100 atmospheres may increase the diffusion rate by atleast about 10 times. Increasing the diffusion rate may promote thenitriding result, for example, for increasing the rate of iron nitrideformation.

The example technique of FIG. 5 also includes depositing shell 14 a or14 b on iron-based core 12 a or 12 b to form core-shell nanoparticle 10a or 10 b (44). Depositing shell 14 a or 14 b on iron-based core 12 a or12 b (44) may include immersing sample holder 32 including iron-basedcore 12 a or 12 b in salt composition 34 for a predetermined period oftime.

In some examples, the technique of FIG. 5 may additionally include,after depositing shell 14 a or 14 b on iron-based core 12 a or 12 b(44), separating nanoparticle 10 a or 10 b from salt composition 34(48). The separating may include screening or filtering, for example,through a screen or a mesh. Nanoparticles 10 a or 10 b may be washedafter the separating to remove any adhering or otherwise residual saltcomposition 34 on nanoparticles 10 a or 10 b.

In some examples, the example technique of FIG. 5 may optionallyinclude, after depositing shell 14 a or 14 b on iron-based core 12 a or12 b (44), drying the core-shell nanoparticles 10 a or 10 b (50). Forexample, the drying may include oven drying, convection drying, orfluidized drying.

The example technique of FIG. 5 also may optionally include, afterdepositing shell 14 a or 14 b on iron-based core 12 a or 12 b (44),nitriding the core-shell nanoparticles 10 a or 10 b (52). For example,the nitriding (52) may be performed similar to or substantially the sameas nitriding iron-based core 12 a or 12 b (42). The nitriding may helpintroduce an iron nitride phase within nanoparticle 10 a or 10 b, forexample, within one or both of core 12 a or 12 b, or shell 14 a or 14 b.In some examples, the technique of FIG. 5 includes only one of steps(42) and (52). In other examples, the technique of FIG. 5 includes bothof steps (42) and (52). In other examples, such as when core-shellnanoparticles 10 a or 10 b do not include iron nitride, the technique ofFIG. 5 may omit both of steps (42) and (52).

The example technique of FIG. 5 may further optionally include, afterdepositing shell 14 a or 14 b on iron-based core 12 a or 12 b (44),annealing nanoparticles 10 a or 10 b (54). For example, the annealingmay include exposing nanoparticles 10 a or 10 b to a magnetic fieldhaving a predetermined strength at a predetermined temperature for apredetermined period of time. For example, the annealing may includeheating the particles to a temperature between about 100° C. and about250° C., such as between about 120° C. and about 220° C., for example,between about 180° C. and 220° C. The annealing process may continue fora predetermined time that is sufficient to allow diffusion of thenitrogen atoms to the appropriate interstitial spaces in the ironcrystal lattice. Such diffusion may promote the formation of ironnitride phases, and may promote the conversion of disordered ironnitride phases, for example, Fe₈N, into ordered iron nitride phases, forexample, Fe₁₆N₂. However, heating at temperatures greater than about250° C. may reduce the formation of ordered iron nitride phases, or maydegrade previously-formed ordered iron nitride phases such as Fe₁₆N₂. Insome examples, the annealing process continues for between about 20hours and about 200 hours, such as between about 40 hours and about 60hours. In some examples, the annealing process may occur under an inertatmosphere, such as Ar, to reduce or substantially prevent oxidation ofthe iron. Further, in some implementations, the temperature is heldsubstantially constant. The annealing may result in magnetic materialincluding at least one α″-Fe₁₆N₂ phase domain.

In some examples, the annealing may include exposing nanoparticles 10 aor 10 b to an external magnetic field during the annealing process.Annealing iron nitride materials in the presence of an applied magneticfield may enhance the Fe₁₆N₂ phase domain formation in iron nitridematerials. Increased volume fractions of α″-Fe₁₆N₂ phase domains mayimprove the magnetic properties of core-shell nanoparticles includingiron nitride. Improved magnetic properties may include, for example,coercivity, magnetization, and magnetic orientation.

In some examples, an applied magnetic field during annealing may be atleast 0.2 Tesla (T). The temperature at which the magnetic fieldannealing is performed may at least partially depend upon furtherelemental additions to the iron nitride base composition and theapproach used to initially synthesize the iron nitride base composition.In some examples, the magnetic field may be at least about 0.2 T, atleast about 2 T, at least about 2.5 T, at least about 6 T, at leastabout 7 T, at least about 8 T, at least about 9 T, at least about 10 T,or higher. In some examples, the magnetic field is between about 5 T andabout 10 T. In other examples, the magnetic field is between about 8 Tand about 10 T. Further details regarding annealing the materialsincluding iron and nitrogen may be found in U.S. Provisional ApplicationNo. 62/019,046, filed Jun. 30, 2014, the entire content of which isincorporated herein by reference.

Thus, the example system of FIG. 4 and the example technique of FIG. 5may be used to form core-shell nanoparticles according to thedisclosure. Other example systems and techniques for forming core-shellnanoparticles are described with reference to FIGS. 6-9.

FIG. 6 is a conceptual and schematic diagram illustrating an examplemilling system 60 for forming example core-shell nanoparticles bymilling. Milling system 10 may be operated in a rolling mode, in which abin 62 of milling system 60 rotates about a horizontal axis of bin 62,as indicated by arrow 69. As bin 62 rotates, milling media 65 (such asmilling spheres, milling bars, or the like) move within bin 62 and, overtime, crush or wear an iron-containing material 64. In some examples,iron-containing material 64 may include iron-based cores 12 a or 12 b.In addition to iron-containing material 64 and milling media 65, bin 12includes a salt composition 66. In some examples, salt composition 66may include one or more precursors of at least one of silica, aluminumoxide, silicon nitride, aluminum nitride, manganese nitride, zinc oxide,iron oxide, ferromanganese, Fe₄N, Fe₈N, or Fe₁₆N₂. Salt composition 66deposits respective shells 14 a or 14 b on iron-based cores 12 a or 12b. For example, salt composition 66 may include one or more of a solid,a slurry, a paste, a suspension, a liquid, a solution, a polymer, or agel.

During the milling, milling media 65 may exert pressure oniron-containing material 64 and salt composition 66, which may result incold welding of material from salt composition 66 onto iron-containingmaterial 64. For example, in examples in which iron-containing material64 includes iron-based core 12 a or 12 b, the milling may result in coldwelding of material from salt composition 66 onto iron-based core 12 aor 12 b, to form a cold welded shell 14 a or 14 b. The cold welding maythus allow shell 14 a or 14 b to be deposited on core 12 a or 12 b,without heating, or without requiring a molten phase of shell material.

In some examples, milling system 60 may include additional components.For example, milling system 60 may include a nitrogen source, fornitriding iron-containing material 64 before, during, or after,depositing shell 14 a or 14 b on iron-based cores 12 a or 12 b. In someexamples, the nitrogen source may include at least one of ammonia,ammonium nitrate, an amide-containing material, or ahydrazine-containing material. In some examples, the amide-containingmaterial may include at least one of a liquid amide, a solutioncontaining an amide, carbamide, methanamide, benzamide, or acetamide.The hydrazine-containing material may include at least one of ahydrazine or a solution containing the hydrazine. In some examples, saltcomposition 66 may include the nitrogen source. In some examples, anitriding composition may include a nitrogen source. In some examples,the nitrogen source may be contained in a dispenser in milling system60.

In some examples, iron-containing material 64 may include an ironprecursor instead of iron-based cores 12 a or 12 b. For example, theiron precursor may include at least one of iron (Fe), FeCl₃, Fe₂O₃, orFe₃O₄. Bin 62 may contain a reducing environment, as described elsewherein the disclosure, and may mill the iron precursor in the reducingenvironment to form iron-based cores 12 a or 12 b. In some examples,milling system 60 may mill the iron precursor with the nitrogen sourceto nitride the iron precursor as iron-based cores 12 a or 12 b areformed.

In the example illustrated in FIG. 6, milling media 65 may include asufficiently hard material that, when contacting iron-containingmaterial 64 with sufficient force, will wear iron-containing rawmaterial 64 and cause particles of iron-containing material 64 to, onaverage, have a smaller size, for example, smaller particles 68. In someexamples, milling media 65 may be formed of steel, stainless steel, orthe like. In some examples, the material from which milling media 65 areformed may not chemically react with iron-containing material 64 and/orthe nitrogen source. In some examples, milling media 65, such as millingspheres, may have an average diameter between about 5 millimeters (mm)and about 20 mm.

To facilitate milling of iron-containing material 64, in some examples,the mass ratio of the total mass of milling media 65 to the total massof iron-containing material 64 may be between about 1:1 to about 50:1,for example, about 20:1.

In some examples, milling system 60 may be used to perform the exampletechnique of FIG. 7. While the example technique of FIG. 7 is describedwith reference to components of milling system 60, the example techniqueof FIG. 7 may be performed with other milling systems. FIG. 7 is a flowdiagram illustrating an example technique for forming core-shellnanoparticle 10 a or 10 b by milling.

In some examples, the technique of FIG. 7 may optionally includereducing an iron precursor to form iron-based cores 12 a or 12 b (70).In some examples, the reducing 70 may be performed similarly to thereducing 40 described with reference to the example technique of FIG. 5.In some examples, the reducing may be performed by milling system 60.For example, bin 62 may mill iron precursor in a reducing environment,as described elsewhere in the disclosure.

In some examples, the example technique of FIG. 7 may optionally includenitriding iron-based core 12 a or 12 b (72). In some examples, thenitriding 72 may be performed similarly to the nitriding 42 describedwith reference to the example technique of FIG. 5. In some examples, thenitriding 72 may be performed using milling system 60.

For example, the nitriding (72) may include milling iron-based core 12 aor 12 b with a nitrogen source for a predetermined period of time. Insome examples, salt composition 66 may include the nitrogen source. Insome examples, a nitriding composition in bin 62 may include thenitrogen source. For example, the nitrogen source may be similar tonitrogen sources described elsewhere in the disclosure.

The technique of FIG. 7 includes depositing shell 14 a or 14 b oniron-based core 12 a or 12 b, by milling iron-based core 12 a or 12 bwith salt composition 66 for a predetermined period of time (74). Forexample, bin 62 of milling system 60 may be rotated at a rate sufficientto cause mixing of the components in bin 62 (e.g., milling media 65,iron-containing material 64, the and salt composition 66) and causemilling media 65 to mill iron-containing material 64 and saltcomposition 66 so as to cause a shell to be deposited from saltcomposition 66 onto iron-based cores 12 a or 12 b in iron-containingmaterial 64. In some examples, bin 62 may be rotated at a rotationalspeed of between about 500 revolutions per minute (rpm) to about 2000rpm, such as between about 600 rpm and about 650 rpm, about 600 rpm, orabout 650 rpm.

In some examples, the depositing 74 may initially include millingiron-based material 64 with one of salt composition 66 or the nitridingcomposition, and may subsequently include milling iron-based material 64with the other of salt composition 66 or the nitriding composition. Insome examples, the nitriding composition may be introduced into saltcomposition 66, or bin 62 may be progressively exposed to the nitrogensource, as the depositing 74 progresses. Eventually, nanoparticle 10 aor 10 b may be formed by respectively depositing shell 14 a or 14 b oncore 12 a or 12 b.

In some examples, the example technique of FIG. 7 may include, after thedepositing 74, separating nanoparticle 10 a or 10 b from saltcomposition 66 (78). The separating 78 may be performed similarly toseparating 48 described with reference to the example technique of FIG.5.

In some examples, the example technique of FIG. 7 may include, after thedepositing 74, drying the core-shell nanoparticles 10 a or 10 b (80).The drying 80 may be performed similarly to the drying 50 described withreference to the example technique of FIG. 5.

In some examples, the example technique of FIG. 7 may include, after thedepositing 74, nitriding the core-shell nanoparticles 10 a or 10 b (82).The nitriding 82 may be performed similarly to the nitriding 52described with reference to the example technique of FIG. 5.

In some examples, the example technique of FIG. 7 may include, after thedepositing 74, annealing nanoparticles 10 a or 10 b (84). The annealing84 may be performed similarly to the annealing 54 described withreference to the example technique of FIG. 5.

Thus, the example system of FIG. 6 or the example technique of FIG. 7may be used to form core-shell nanoparticles according to thedisclosure. Other example systems and techniques for forming core-shellnanoparticles are described with reference to FIGS. 8 and 9.

FIG. 8 is a flow diagram illustrating an example technique for treatinga composition including an iron-based material. For example, the exampletechnique of FIG. 8 includes treating a composition including core-shellnanoparticle 10 a or 10 b. In some examples, the composition may includeany iron-based material, for example, elemental iron, iron nitride,α″-Fe₁₆N₂, or α′-Fe(Co)(N). In some examples, the iron-based materialmay include iron-based workpieces. For example, workpieces may includegrains or powders, such as spheres, cylinders, flecks, flakes, regularpolyhedra, irregular polyhedra, and any combination thereof. Examples ofsuitable regular polyhedra include tetrahedrons, hexahedrons,octahedron, decahedron, dodecahedron and the like, non-limiting examplesof which include cubes, prisms, pyramids, and the like. In someexamples, workpieces may include a dimension that is longer, e.g., muchlonger, than other dimensions of the workpiece. Example workpieces witha dimension longer than other dimensions include fibers, wires,filaments, cables, films, thick films, foils, ribbons, sheets, or thelike. In other examples, workpieces may not have a dimension that islonger than other dimensions of the workpiece. In some examples,workpieces may include nanoparticles, for example, core-shellnanoparticles. The workpieces may be further processed to form bulkmagnets. For example, the workpieces may be sintered, bonded, or bothsintered and bonded together directly to form a bulk magnet.

In some examples, the example technique of FIG. 8 includes autoclavingthe composition at a predetermined pressure, at a predeterminedtemperature, for a predetermined period of time, in an environmentincluding a nitrogen source (90). Instead of, or in addition to,autoclaving, the example technique of FIG. 8 may include any suitablehigh-pressure nitriding step. In some examples, the autoclaving 90includes heating the composition to a temperature greater than about200° F. In some examples, the autoclaving 90 includes, simultaneouslywith the heating, pressurizing the composition to a pressure betweenabout 1 atmospheres and about 100 atmospheres, or greater than 100atmospheres. The autoclaving 90 nitrides the composition, for example,by introducing an iron nitride phase in one or both of core 12 a or 12 band shell 14 a or 14 b of nanoparticle 10 a or 10 b. Without wishing tobe bound by theory, diffusion of nitrogen species increases withpressure. Increasing the pressure, for example, by autoclaving,increases nitrogen diffusion. Using a pressure of at least about 100atmospheres may increase the diffusion rate by at least about 10 times.Increasing the diffusion rate may promote the nitriding result, forexample, for increasing the rate of iron nitride formation.

In some examples, the example technique of FIG. 8 may include millingthe composition with the nitrogen source for a predetermined period oftime. The milling may be performed similar to the milling 74 describedwith reference to FIG. 7.

In some examples, the nitrogen source used in the autoclaving (90) orthe milling includes at least one of ammonia, ammonium nitrate, anamide-containing material, or a hydrazine-containing material. In someexamples, the amide-containing material includes at least one of aliquid amide, a solution containing an amide, carbamide, methanamide,benzamide, or acetamide. In some examples, the hydrazine-containingmaterial includes at least one of a hydrazine or a solution containingthe hydrazine.

In some examples, the example technique of FIG. 8 may optionallyinclude, after the autoclaving (90) or the milling, annealing thedeposited iron-based core (92). The annealing 92 may be performedsimilarly to the annealing 54 described with reference to the exampletechnique of FIG. 5.

Thus, the example technique of FIG. 8 may be used to treat a compositioncomprising iron-based materials, for example, iron-based workpiecesincluding core-shell nanoparticles, for example, to nitride iron-basedmaterials to introduce an iron nitride phase.

EXAMPLES Example 1

The coercivity H_(C) of ellipsoidal particles of Fe₁₆N₂ was calculatedfor varying particle dimensions, using the equation H_(C)=H_(S) (1−P).The shape anisotropy H_(S)=4π.ΔN.M_(S), where M_(S) is the saturationmagnetization (200 emu/g for Fe₁₆N₂). ΔN=N_(a)−N_(c) is thedemagnetizing factor difference between longitude and transversedirections of the particles. The packing factor P=0.52 for close simplecubic packed ellipsoids. The coercivity depended on the ratio of thelength of the particles (c) to the diameter of the particles (a), as setforth in TABLE 1.

TABLE 1 c/a ΔN H_(S) (Oe) H_(C)(Oe) 2 0.24 4524 2171 3 0.34 6409 3076 70.45 8482 4072

Example 2

The coercivity H_(C) of ellipsoidal particles of Fe₁₆N₂ including anon-magnetic shell of thickness h was calculated for varying particledimensions, where a, b, and c are the three ellipsoidal diameters, usingthe. The packing factor P was calculated using EQUATION 1.

$\begin{matrix}{\ {P = \frac{\frac{4\;\pi}{3} \cdot a \cdot b \cdot c}{\left( {{2\; a} + {2\; h}} \right) \cdot \left( {{2\; b} + {2\; h}} \right) \cdot \left( {{2\; c} + {2\; h}} \right)}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The coercivity depended on a, b, c, and h, as set forth in TABLE 2.

TABLE 2 H_(C) (Oe) H_(C) (Oe) H_(C) (Oe) H_(C) (Oe) a = 60 nm, a = 60nm, a = 60 nm, a = 60 nm, H_(S) H_(C) b = 60 nm, b = 60 nm, b = 60 nm, b= 60 nm, c/a ΔN (Oe) (Oe) h = 3 nm h = 6 nm h = 12 nm h = 20 nm 2 0.244524 2171 2429 2660 3029 3382 3 0.34 6409 3076 3416 3727 4225 4710 70.45 8482 4072 4484 4865 5486 6099

Example 3

Example core-shell nanoparticles were prepared by sputtering ananti-ferromagnetic MnN shell on ellipsoidal Fe₁₆N₂ cores. Themagnetization behavior of the example core-shell nanoparticles wasstudied. FIG. 9 is a diagram illustrating a hysteresis loop ofmagnetization versus applied magnetic field at room temperature forexample nanoparticles including Fe₁₆N₂ cores and MnN shells. Arelatively high coercivity of about 1605 Oe was achieved.

Example 4

Example core-shell nanoparticles were prepared by ammonia nitridingellipsoidal Fe particles. Fe₁₆N₂ initially formed as a thin shell at thesurfaces of reduced Fe particles, and the thickness of the Fe₁₆N₂increased as a function of nitriding time, to eventually form Fe/Fe₁₆N₂core-shell nanoparticles. FIG. 10 is a diagram illustrating the x-raydiffraction data for the example nanoparticles including Fe cores andFe₁₆N₂ shells, for sample FNP218. The example nanoparticles of FIG. 10had a ratio C_(Fe16N2)/C_(Fe)=0.72. FIG. 11 is a diagram illustrating ahysteresis loop of magnetization versus applied magnetic field at roomtemperature for the example nanoparticles of FIG. 10. A coercivity ofabout 1010 Oe and a magnetization saturation of about 150 emu/g wereobserved. FIG. 12 is a diagram illustrating the x-ray diffraction datafor the example nanoparticles including Fe cores and Fe₁₆N₂ shells, forsample FNP224, after nitriding for 50 hours using ammonia. FIG. 13 is adiagram illustrating a hysteresis loop of magnetization versus appliedmagnetic field at room temperature for the example nanoparticles of FIG.12. A coercivity of about 650 Oe and a magnetization saturation of about50 emu/g were observed. As shown in FIGS. 10 and 12, the saturationmagnetization decreased to about 50 emu/g from about 150 emu/g as theshell was developed into the core.

Example 5

The effect of field annealing on FeCo/MnN core-shell particles, withFeCo cores, and MnN shells, was investigated. The particles were cappedwith Ta. FIG. 14 is a diagram illustrating the effect of the fieldannealing on a hysteresis loop of magnetization versus applied magneticfield at room temperature for the example nanoparticles including FeCocores and MnN shells. Both the coercivity H_(C) and the saturationmagnetization M_(S) were increased by the annealing, as shown in FIG.14.

Example 6

The magnetization behavior of Fe nanoparticles was compared to that ofcore-shell nanoparticles having Fe cores and MnN shells, at 10K and at300K. Both kinds of nanoparticles were Ta capped. FIG. 15 is a diagramillustrating a hysteresis loop of magnetization versus applied magneticfield at room temperature (300K) and low temperature (10K) for theexample Fe nanoparticles. FIG. 16 is a diagram illustrating a hysteresisloop of magnetization versus applied magnetic field at room temperature(300K) and low temperature (10K) for the example core-shellnanoparticles having Fe cores and MnN shells. As shown in FIGS. 15 and16, the coercivity increased on using MnN shells.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A nanoparticle comprising: an iron-based core,wherein the iron-based core comprises elemental iron and α″-Fe₁₆N₂; anda shell, wherein the shell comprises an anti-ferromagnetic material;wherein the nanoparticle has a coercivity of at least about 600 Oe. 2.The nanoparticle of claim 1, wherein the shell comprises at least one ofmanganese nitride or ferromanganese.
 3. The nanoparticle of claim 1,wherein the core has a major dimension between about 20 nm and about 100nm.
 4. The nanoparticle of claim 1, wherein the shell has a thicknessbetween about 1 nm and about 10 nm.
 5. The nanoparticle of claim 1,wherein the core is an ellipsoid with a ratio of a maximum diameter to aminimum diameter of at least about
 2. 6. A bulk magnetic materialcomprising a plurality of the nanoparticles of claim
 1. 7. Thenanoparticle of claim 1, formed by depositing a shell on an iron-basedcore to form a core-shell nanoparticle by at least milling theiron-based core with a salt composition for a predetermined period oftime; wherein the salt composition comprises at least one of precursorsof silica, aluminum oxide, silicon nitride, aluminum nitride, manganesenitride, zinc oxide, iron oxide, ferromanganese, Fe₄N, and Fe₁₆N₂. 8.The nanoparticle of claim 7, wherein the salt composition comprises atleast one of a solid, a slurry, a paste, a suspension, a liquid, asolution, a polymer, or a gel.
 9. The nanoparticle of claim 7, whereinthe nanoparticle is formed by a method further comprising, afterdepositing the shell: filtering the core-shell nanoparticle from thesalt composition, and drying the core-shell nanoparticle.
 10. Thenanoparticle of claim 7, wherein the nanoparticle is formed by a methodfurther comprising, after depositing the shell, nitriding the core-shellnanoparticle.
 11. The nanoparticle of claim 10, wherein nitriding thecore-shell nanoparticle comprises autoclaving the core-shellnanoparticle at a predetermined pressure, at a predeterminedtemperature, for a predetermined period of time, in a nitrogen-richenvironment, wherein the predetermined pressure is greater than 100atmospheres.
 12. The nanoparticle of claim 11, wherein the nitrogen-richenvironment is generated by a nitrogen source comprising at least one ofammonia, ammonium nitrate, an amide-containing material, or ahydrazine-containing material.
 13. The nanoparticle of claim 7, whereinthe nanoparticle is formed by a method further comprising, afterdepositing the shell, annealing the core-shell nanoparticle in thepresence of an applied magnetic field, wherein the annealing thecore-shell nanoparticle in the presence of an applied magnetic fieldcomprises exposing the core-shell nanoparticle to a magnetic fieldhaving a predetermined strength at a predetermined temperature for apredetermined period of time.